The present invention relates to a laminate type organic electrophotographic receptor used in an image forming device such as a copying machine, a printer or a facsimile, and especially relates to an improved coating fluid for forming an improved electric charge generating layer and the organic electrophotographic receptor formed using the coating fluid, and the method of manufacturing the same.
Heretofore, along with the improvement in developing the material for an electrophotographic receptor, the material for the receptor has changed from inorganic materials such as zinc oxide, cadmium sulfide, selenium amorphous and amorphous silicon, to organic photoconductive materials (OPC). The electrophotographic receptor formed using the organic photoconductive material has some problems related to its sensitivity, durability and environmental stability, but it has greater advantages compared to the inorganic material in its toxicity, cost, and freedom of material design and the like.
Therefore, many proposals for improving the sensitization of the organic electrophotographic receptor exist.
There are two types of photoconductive layer of the organic electrophotographic receptor, a single layered type and a laminated type. Since the laminate type receptor (hereinafter called xe2x80x9cseparated-function receptorxe2x80x9d) composed mainly of a layer (hereinafter called xe2x80x9ccharge generating layer:CGLxe2x80x9d) including a material that generates a charge carrier when exposed to light (hereinafter called xe2x80x9ccharge generating material:CGMxe2x80x9d) and a layer (hereinafter called xe2x80x9ccharge transferring layer:CTLxe2x80x9d) mainly formed of a material that receives the charge carrier generated at the CGL and transfers the same (hereinafter called xe2x80x9ccharge transferring material:CTMxe2x80x9d) is known to provide advantageous sensitization, it constitutes the main portion of organic receptors that are currently utilized. Further, since recent studies have improved the durability of the above-mentioned laminated receptor, it is considered to become the mainline of receptors in the near future.
Moreover, the durability of the receptor is further improved by providing an undercoating layer (hereinafter called xe2x80x9cUCLxe2x80x9d) on the conductive base so as to improve the electrification, to prevent unnecessary charge injection from the conductive base, to cover the defects on the surface of the conductive base, to prevent the creation of pinholes, and to improve the adhesiveness of the photosensitive layer.
These photosensitive layers are formed by creating a coating fluid including photosensitive material by dissolving or dispersing the organic photoconductive material for each layer together with a binding resin in an organic solvent, and applying and drying the coating fluids including the photosensitive material on the conductive base in order.
The method for applying the organic electrophotographic photosensitive layer includes a spraying method, a bar-coat method, a roll coat method, a blade method, a ring method, and a dip coating method. The dip coating method is a method where the (cylindrical) conductive base is immersed into a coating tank (filled with the photosensitive material) and then pulled out either at constant speed or at varying speeds set freely so as to form the photosensitive layer. This dip coating method is relatively easy and advantageous in its productivity and its low cost, so it is often used when manufacturing the electrophotographic receptor.
One example of the device used for dip coating is shown in FIG. 1.
A coating fluid 5 including the photosensitive material is stored in a coating tank 4. A cylindrical conductive base 1, the upper end of which is supported in an airtight manner by a chucking device 8, is immersed into the coating solution 5 of the coating tank 4. Upon dipping, the chucking device 8 is lowered by an elevating machine 2 equipped with a motor 3, and the base 1 is immersed into the coating fluid 5. After enough dipping has been performed, the chucking device 8 is raised by the elevator 2. The elevator 2 enables the base 1 to be immersed into the coating tank for a desired depth by controlling and confirming the rotation of the motor 3. According to another example, the coating tank can be elevated when performing the coating.
Upon dipping, the fluid that has overflown from the tank 4 is collected into an auxiliary tank as shown in arrow 13, and the coating fluid is adjusted so as to maintain a constant viscosity using a viscometer 16 and a fluid adding device 10. After stirring the fluid with a stirring device (impeller) 12, foreign matter in the fluid is removed through a filter 9, and as shown in arrow 14, the fluid is returned to the coating tank by a pump 6, and the tank 4 is filled with the coating fluid 5 before repeating the next coating steps.
In order to realize supersensitivity in a separated-function type laminated receptor, not only is it necessary to use a charge generating material having high quantum efficiency, but it is also necessary to form a very thin charge generating layer in order to increase the ratio of charge generating material within the charge generating layer or to move the electrons toward the substrate faster, the speed of the electrons being very slow compared to the speed in which the holes move toward the charge transferring layer.
However, during the application of the electric charge generating layer, external defects can be generated such as the unevenness of the film thickness, the ring-like stripes, the fluid droppings, and the belt-like liquid pool formed on the lower end of the base. Further, since the coating fluid for the charge generating layer is a pigment dispersion fluid, if the dispersion is of poor quality, the pigments can aggregate and create black dots or white dots.
These defects are caused by the fouling of the base body, the uneven dispersion of the application fluid, or the convection within the coated layer when the coated solvent is evaporating. Therefore, the charge generating material can be in some locations while not being in other areas of the layer. Such unevenness of the coated layer causes fatal image defects especially in the charge generating layer of the supersensitive receptor required to be thin, and deteriorates the quality of the image forming device.
Japanese Patent Laid-Open Publication No. 2-203348, Japanese Patent Laid-Open Publication No. 4-14053, and Registered patent Publication No. 2853336 disclose methods for improving the coating fluid for the charge generating layer in order to overcome the above-mentioned problems. These references disclose a method for using and mixing two kinds of organic solvents as the coating fluid (each using dioxane/cyclohexanone, low boiler/high boiler solvents, or low viscosity/high viscosity solvents), and the references all insist that these. solvents improve the dispersibility, the stability and the application performance of the coating fluid.
However, in the case of the coating fluid formed of more than two kinds of mixed solvents, the difference in the boiling point, the vapor pressure, the vapor rate and the like of the different solvents causes the quantity of evaporation of each solvent to differ, causing the composition ratio including the mixed solvent ratio within the coating fluid to vary. If the composition ratio of the mixed solvents is varied, the quality of the coated layer can be deteriorated by the change in color or forming of dews on the layer, or the dispersibility or solubility of the solution can be deteriorated.
Moreover, though the prior art references insist that the coating unevenness is solved by their inventions, since they mix different solvents that have different properties, the applicability is deteriorated and coating unevenness is still caused by the change or bias in surface tension when the solvents in the applied layer evaporate.
Therefore, other references (refer to Japanese Patent Laid-Open Publication Nos. 6-208230 and 7-295247) disclose method of including polydimethylsiloxane in the coating fluid for forming the charge generating layer.
The technique disclosed in the cited references aim at improving the dispersibility, the stability and the application performance of the coating fluid by including a polydimethylsiloxane in the coating fluid of the charge generating layer in a laminate type receptor comprising at least an electric charge generating layer and an electric charge transferring layer.
However, according to the recent trend of cutting down the manufacture cost, an undercoating layer is often formed since a base tube having a rough surface roughness is used in creating the receptor. In a receptor comprising an undercoating layer that is required to have supersensitivity, the charge generating layer must be formed very thin, so the techniques disclosed in the references are not enough to solve the problem of the charge generating layer with defects caused by the uneven coating of the undercoating layer surface.
The present invention aims at solving the above-mentioned problems of the prior art. The object of the present invention is to provide a coating fluid having good dispersibility, improved stability and good application property for forming the electric charge generating layer that improves the supersensitivity of the laminate type separated-function receptor, and thereby enables to provide a stable and improved organic electrophotographic receptor at a low cost.
The present invention relates to a coating fluid for forming an electric charge generating layer of an organic electrophotographic receptor comprising at least an undercoating layer, an electric charge generating layer, and an electric charge transferring layer which are laminated sequentially, said coating fluid being composed of at least an electric charge generating material, a binding resin, a silicon oil having a surface tension of 22 mN/m or less, and an organic solvent.
According to the present invention where silicon oil is included in the coating fluid for forming the electric charge generating layer, the surface tension of the coating material is reduced, and therefore the wetting characteristic of the fluid is improved by the improved dispersibility of the pigments and the reduced interfacial energy difference with the undercoating layer surface. Therefore, according to the invention, the dispersibility, the stability and the coating ability of the coating fluid is improved. The present invention prevents the occurrence of deteriorated exteriority such as layer unevenness, ring-like stripes, fluid dripping, belt-like fluid pool formed at the lower end of the base, or black/white dots formed by the pigments being gathered due to bad dispersion.
Moreover, the present invention characterizes especially in using a silicon oil having a surface tension of 22 mN/m or less in the coating fluid for electric charge generating layer of an organic electrophotographic receptor comprising at least an undercoating layer, an electric charge generating layer, and an electric charge transferring layer which are laminated in this order. Therefore, in a receptor comprising an undercoating layer utilizing a base tube having a rough surface so as to cut down the cost etc., if there is a need for supersensitivity and the electric charge generating layer must be formed very thin, the coating material will not be influenced by the interfacial energy of the undercoating layer surface, and coating unevenness is prevented. In contrast, if the surface tension of the silicon oil is greater than the maximum value set according to the present invention, the interfacial energy difference becomes great and the leveling effect decreases, thereby causing coating unevenness.
According further to the present invention, the viscosity of the silicon oil is 50 cs or less.
According to this feature of the invention, when the coating fluid for electric charge generating layer aims at providing supersensitivity, the viscosity of the coating fluid must be set low, generally in the range of 1 to 30 cs, so as to form a thin layer. Therefore, the present invention limits the viscosity of the silicon oil to 50 cs or less, setting a sufficiently small viscosity difference with the coating fluid, which brings out the leveling effect of the silicon oil sufficiently. Further, this effect is maintained for a long time, preventing the occurrence of deteriorated exteriority such as layer unevenness, ring-like stripes, fluid dripping, belt-like fluid pool formed at the lower end of the base, or black/white dots formed by the pigments being gathered due to bad dispersion. In contrast, when the viscosity exceeds the range set according to the present invention, the aimed effect is obtained at first, but as time passes the coating fluid and the silicon oil are separated, and the same effects can no longer be provided in long term.
Moreover, the coating fluid for forming an electric charge generating layer of an organic electrophotographic receptor according to the invention is characterized in that the amount of the silicon oil being added is in the range of 1-30 weight % against said binding resin.
By setting the added amount of silicon oil to be in the range of 1 to 30 weight % against the binding resin, the present invention enables. to constantly prevent the occurrence of deteriorated exteriority such as layer unevenness, ring-like stripes, fluid dripping, belt-like fluid pool formed at the lower end of the base, or black/white dots formed by the pigments being gathered due to bad dispersion, without deteriorating the electric characteristics of the photoreceptor. If the amount is less than the range set according to the invention, the aimed leveling effect is not sufficiently exerted, and when the amount is greater than the set range, the rise potential increases during repeated use, making it difficult to maintain an appropriate image density.
Even further, the coating fluid for forming an electric charge generating layer of an organic electrophotographic receptor characterizes in that the silicon oil is polydimethylsiloxane.
According to the invention where the silicon oil is polydimethylsiloxane, the dispersibility, the stability and the coating ability of the coating fluid is improved. Therefore, the present invention enables to prevent the occurrence of deteriorated exteriority such as layer unevenness, ring-like stripes, fluid dripping, belt-like fluid pool formed at the lower end of the base, or black/white dots formed by the pigments being gathered due to bad dispersion.
Further, the present invention characterizes in that the silicon oil is polymethylphenylsiloxane.
According to the invention where the silicon oil is polymethylphenylsiloxane, the dispersibility, the stability and the coating ability of the coating fluid is improved. Therefore, the present invention enables to prevent the occurrence of deteriorated exteriority such as layer unevenness, ring-like stripes, fluid dripping, belt-like fluid pool formed at the lower end of the base, or black/white dots formed by the pigments being gathered due to bad dispersion.
Even further, the present invention characterizes in that the electric charge generating material is an oxotitanyl phthalocyanine crystal that shows a maximum diffraction peak at a Bragg angle (2xcex8xc2x10.2xc2x0) of 9.4xc2x0 or 9.7xc2x0 and further show clear diffraction peaks at least at 7.3xc2x0, 9.4xc2x0, 9.7xc2x0 and 27.30xc2x0 according to an X-ray diffraction spectrum.
According to the invention, the coating fluid for the charge generating layer comprising an oxotitanyl phthalocyanine crystal that shows a maximum diffraction peak at a Bragg angle (2xcex8xc2x10.2xc2x0) of 9.4xc2x0 or 9.7xc2x0, and further show clear diffraction peaks at least at 7.3xc2x0, 9.4xc2x0, 9.7xc2x0 and 27.3xc2x0 according to an X-ray diffraction spectrum provides the best coating performance. Moreover, the material fluid enables to create a constantly stable and uniform coating layer for a long time.
Further, the present invention characterizes in that the binding resin is a butyral resin.
According to the invention where the coating fluid for the charge generating layer comprises a butyral resin as the binding resin, the coating fluid provides great electric characteristics. Moreover, the fluid enables to create a constantly stable and uniform coating layer for a long time.
Even further, the present invention characterizes in that the organic solvent is a non-halogen series organic solvent, and is further a mixture of two or more kinds of non-halogen series organic solvents.
According to the present invention utilizing a non-halogen series organic solvent, a safe method for manufacturing receptors is provided having no bad effect on the environment or on the operators. Further, when more than two kinds of mixed solvents are used to bring out the performance of the charge generating material sufficiently, the density unevenness that may be cause during the drying process due to the difference in the evaporation speed/boiling point/vapor pressure/surface tension/relative density etc. of the different solvents is prevented according to the invention.
Further, the present invention characterizes in that the organic solvent is a mixture of dimethoxyethane/cyclohexanone.
According to the present invention related to a coating fluid for forming a charge generating layer especially comprising an oxotitanyl phthalocyanine crystal that shows a maximum diffraction peak at a Bragg angle (2xcex8xc2x10.2xc2x0) of 9.40xc2x0 or 9.7xc2x0, and further show clear diffraction peaks at least at 7.3xc2x0, 9.4xc2x0, 9.7xc2x0 and 27.3xc2x0 according to an X-ray diffraction spectrum, and with a mixed solvent composition providing the best performances, it is possible to provide a constantly stable and uniform coating layer during the whole lifetime of the coat material.
Even further, the present invention provides an organic electrophotographic receptor having an electric charge generating layer formed on a cylindrical conductive base, wherein said electric charge generating layer is formed by dip coating a coating fluid composed at least of an electric charge generating material, a binding resin, a silicon oil having a surface tension of 22 mN/m or less, and an organic solvent, and further provides a method for manufacturing an electric charge generating layer on a cylindrical conductive base by dip coating the coating fluid according to the present invention.
According to the present invention where silicon oil is included in the coating fluid for forming the charge generating layer, the dispersibility, the stability and the coating ability of the coating fluid is improved. Therefore, the invention enables to provide an organic electrophotographic receptor while preventing the occurrence of deteriorated exteriority such as layer unevenness, ring-like stripes, fluid dripping, belt-like fluid pool formed at the lower end of the base, or black/white dots formed by the pigments being gathered due to bad dispersion.
According to yet another aspect, the present invention provides a coating fluid for forming an electric charge generating layer of an organic electrophotographic receptor comprising at least an undercoating layer, an electric charge generating layer, and an electric charge transferring layer which are laminated in said order, wherein the coating fluid includes a silicon oil having a surface tension of 22 mN/m or less. Therefore, in a receptor comprising an undercoating layer utilizing a base tube having a rough surface so as to cut down the cost etc., if there is a need for supersensitivity and the electric charge generating layer must be formed very thin, the present invention provides a coating fluid that effectively prevents coating unevenness, and enables to provide an organic electrophotographic receptor suited for supersensitivity that is constantly even and can be manufactured at low cost.